A new species of nematode, Iotonchus aequatorialis sp. n., found in a soil sample from Ecuador, is described and illustrated. Iotonchus aequatorialis sp. n. is a species of medium length (2.24–2.36 mm) with a large, barrel-shaped mouth cavity 57.5–65 μ m long and 32.5–38.5 μ m diam., basal dorsal tooth, didelphic female reproductive apparatus with short, symmetrical, reflexed ovaries, relatively short vagina provided with sclerotised pieces, large uterus, no sphincter between uterus and oviducts, paravulval papillae absent, a long, slender tail 8–10 times as long as the anal body diam., and a terminal spinneret. The identities of the genera Iotonchus Cobb, 1916 and Iotonchulus Andrássy, 1994 are discussed, new diagnoses of the two genera given and their species listed. A key to the species of Iotonchus is provided. Iotonchulus darreni n. comb. (= Iotonchus darreni n. syn.) is proposed.